1. Field of the Invention
This invention relates generally to crosswell seismic imaging and more particularly to an improved method for generating crosswell maps, or tomograms, representing a property of the subsurface structure.
2. Description of the Prior Art
In the field of geophysics, the knowledge of the subsurface structure of the earth is useful for finding and extracting mineral resources such as oil and natural gas. Techniques which have been utilized for obtaining knowledge of the subsurface include surface seismic exploration, crosswell seismic tomography and well logging.
Surface seismic exploration produces data which cover a large volume of the earth's subsurface, however, data resolution is low. The maximum utilizable seismic frequencies are several hundred Hz., and the resulting spatial resolution is correspondingly limited. Wireline logs provide highly detailed information, but coverage is limited to the well locations. Crosswell seismic tomography is an important tool for bridging the gap between surface seismic data and wireline log data. Crosswell seismic tomography provides data for the earth's subsurface extending between well locations, and it provides this data at a higher resolution than is provided by surface seismic data.
Conventional crosswell seismic imaging typically utilizes a pair of boreholes in proximity to a reservoir of interest. In the first of these boreholes, a seismic source is deployed for emitting seismic energy into the region of interest, often as a swept frequency signal extending through a selected frequency range. The source is sequentially moved through a series of positions within the first borehole and a seismic signal is generated at each position. The seismic energy passes through the subterranean formation of interest to the second one of the pair of boreholes. A receiver array is typically deployed within the second borehole and, like the seismic source, the receiver array is moved through a series of positions within the second borehole. By transmitting a signal from each source position in the first borehole and receiving data from each source position at each receiver position in the second borehole, a seismic crosswell data set is generated. Surveys may also be conducted across a region penetrated by a plurality of boreholes by deploying a source in one of the boreholes and deploying receivers in each of a plurality of boreholes so as to simultaneously record a plurality of data sets.
After having generated a seismic data set, the task of using the data set to produce a crosswell image or tomogram may be undertaken. A number of models have been developed in the prior art for producing tomograms from the seismic crosswell data. The conventional modeling scheme assumes that there are two vertical boreholes and a vertical plane extending between the two boreholes. However, a more recently developed model, as disclosed in U.S. patent application Ser. No. 09/152,935, filed on Sep. 14, 1998, by Washbourne et al., and assigned to TomoSeis, Inc., is a three dimensional model. The Washbourne et al. patent application also discloses a method of handling data from boreholes which exhibit significant deviations from the vertical direction. U.S. patent application Ser. No. 09/152,935, filed on Sep. 14, 1998, is incorporated herein by reference for all purposes.
The data records from a typical crosswell survey represent a very large body of information. For example, if data are obtained from three hundred different receiver positions and each receiver position receives data from each of three hundred source positions, the result will be ninety thousand separate data records or "traces". Crosswell imaging contemplates the use of this data to produce a map representing a seismic parameter, such as velocity, of the subsurface structure in the vicinity of the wellbores.
In general, the subsurface structure is mathematically modeled and this model is used as a basis for forming a tomographic image of a seismic parameter of interest, such as velocity. In one model which is typically used, the vertical plane extending between two wellbores is divided into square pixels and the region within a pixel is assumed to by homogeneous with regard to seismic properties such as wave propagation velocity. A system of equations is set up, based on the travel times inferred from the crosswell data of raypaths extending between source and receiver locations, and the distances traversed within each pixel by the various raypaths. This system of equations is then solved to determine the velocity profile within the subsurface structure between the wellbores.
Another method utilizes geological formation boundaries, such as formation tops, which have been identified from well logging data, or other data as a basis for forming the model, which may be a three dimensional model. Data representing the identified formation tops are applied to a 2-D Chebyshev polynomial, and subterranean surfaces are then defined which approximate the interfaces between subsurface strata of differing lithology. Additional surfaces, extending laterally between the surfaces calculated from the formation tops, are then added to the model. A seismic property of interest, such as velocity, of the subsurface region between the surfaces is then modeled with another Chebyshev polynomial. A system of equations is then set up, based on the travel times inferred from the crosswell data of raypaths extending between source and receiver locations, and the distances traversed within each layer, and this system of equations is solved to determine the velocity profile within the subsurface structure between the wellbores in a manner which is substantially analogous to the method utilized with the pixel based model.
It is well established in the literature that improved resolution will be achieved by an increase in the aperture angle of the transmission paths from source to receiver locations, however, it has also been proposed in the article authored by Spyros K. Lazaratos and Bruce P. Marion, Crosswell seismic imaging of reservoir changes caused by CO.sub.2 injection, The Leading Edge, September 1997, pp. 1300-1306, to delete raypaths having low offset angles (source depth minus receiver depth). For horizontal layering which is substantially horizontal, the raypaths having low offset angles will travel subparallel (nearly parallel) to the layering, and for even moderate velocity contrasts between the layers, the first arrivals will generally be headwaves, which have traveled along the surface interface between the geological layers at least a part of the distance between the source and receiver locations. Headwave arrivals often have very little energy and the waveform of the arriving signal may be very complex because of the interaction between the head waves and other types of arrivals. This complexity can result in ambiguous picks for the first arrival times, and the inclusion of low offset raypaths in the data set can diminish the quality of the resulting tomogram. In the example discussed in said Lazaratos and Marion article, traveltimes corresponding to raypaths forming angles of less than 20 degrees with the horizontal were not used.
Further background information of interest is included in the following articles:
Phillips, W. S. and Fehler, M. C., 1991, Traveltime tomography: A comparison of popular methods: Geophysics, 56, no.10, 1639-1649. PA0 Guiziou, J. L., Mallet, J. L. And Madariaga, R., 1996, 3-D seismic reflection tomography on top of the GOCAD depth modeler: 61, 5, no. 1, 1499-1510 PA0 Chiu, S. K. L., and Stewart, R. R., 1987, Tomographic determination of three-dimensional seismic velocity structure using well logs, vertical seismic profiles, and surface seismic data, Geophysics, 52, 8, 1085-1098. PA0 Dynes and Lytle, 1979, Computerized geophysical tomography, Proc IEE, 67, 1065-1073. PA0 Scales, J.,1987, Tomographic inversion via the conjugate gradient method, Geophysics, 52, 179-185. PA0 Scales, J., Doherty, P., and Gersztenkorn, A., 1990, Regularization of nonlinear inverse problems: imaging the near surface weathering layer: Inverse Prob, 6, 115-131. PA0 Gersztenkorn, A. and Scales, J., 1987, Smoothing seismic tomograms with alpha-trimmed means: Geophys. J. R. Astron. Soc., 91, 67-72. PA0 Meyerholtz, K. A., Pavlis, G. L., and Szpanowski, S. A., 1989, Convolutional quelling in seismic tomography, Geophysics, 54, 570-580.